A cellular radio system typically comprises a fixed base station network, and a plurality of subscriber stations, each communicating with one or more of its base stations. A base station forwards traffic received from a subscriber station. By means of the base stations, the subscriber stations may send messages both to one another and to the subscriber stations of other telephone systems when they are within the coverage area of the base station network, whether they are moving or stationary. In order that a subscriber station can use the services offered by the cellular radio system, the subscriber station must maintain a connection to at least one base station in all circumstances. When a subscriber station does not use the services of the base station network, no connection is required, but each subscriber station listens to one or more of the base stations when the respective subscriber station is in an idle mode. When the subscriber station travels within the base station network from the coverage area of one base station to that of another base station, a need arises for performing a handoff.
In a typical cellular radio system, the subscriber station only communicates with one base station at a time, although in the CDMA system in particular, the subscriber station may simultaneously communicate with a plurality of base stations. In a soft handoff in accordance with prior art, the connection to the base station network remains uninterrupted despite the handoff. In this kind of handoff, the base station is typically changed. Another handoff technique in accordance with prior art is softer handoff, in which the base station is not changed, but the sector of the base station used is changed. Soft and softer handoffs are termed as make-before-break handoffs, which indicates that a new connection is established for the subscriber station before terminating the old connection. In both handoff techniques, the frequency band used remains the same.
In cellular radio systems, so-called hard handoff is also used, which is a break-before-make handoff. This means that a new connection from a subscriber station to a new base station is established only after releasing the connection to an old base station. Although the CDMA system, in particular, has been designed to operate supported by soft and softer handoffs, there are several reasons in cellular radio systems for performing a hard handoff. These reasons may be, e.g., a need to change the usage parameters, the frequency band used, the radio system, etc.
When performing a hard handoff in prior art solutions, it is not possible to determine (i.e. transfer to the subscriber station) all the usage parameters related to the new connection before the handoff. In the existing solutions, it is not possible either, to return to use the previous connection in case the hard handoff fails. Furthermore, a subscriber station cannot be used for performing a hard handoff with the assistance of another subscriber station. A hard handoff cannot be assisted by any other part of the of the cellular radio system, either. These lacking features are obvious drawbacks related to the prior art radio systems.